Ultracold polyatomic molecules are promising candidates for quantum science experiments and sophisticated physics studies beyond the Standard Model, but a key requirement is the ability to achieve full quantum control of the molecule's internal structure.
In view of this, Loïc Anderegg of Harvard University and Nicholas R. Hutzler of Caltech established coherent control of individual quantum states in calcium hydroxide (CaOH) and demonstrated a method for searching the electron electric dipole moment (eEDM). method. Light-trapping ultracold CaOH molecules are prepared in a single quantum state, polarized in an electric field, and coherently transferred to an eEDM sensitive state for electron spin precession measurement. To extend the coherence time, the authors used eEDM-sensitive states with tunable, near-zero magnetic field sensitivity. The results of this work establish a path for eEDM searches using trapped polyatomic molecules.
Experimental methods for polyatomic molecular control
The authors demonstrate complete quantum control of the internal states of polyatomic molecules trapped in vibrational bending modes with high polarizability in small electric fields. Ultracold, optically trapped molecules are first prepared at the individual ultrafine level, and then an electrostatic field is applied to polarize the molecules. The authors observe spin precession in a range of electric and magnetic fields and describe the limitations of current measurements of coherence times. With readily available experimental parameters, coherence times on the order of state lifetimes (>100 ms) can be practically achieved, implementing the key components of eEDM measurements in this system.
Experimental Overview and Singlet Preparation
The experiment first loaded laser-cooled CaOH molecules from the magneto-optical trap into the optical dipole trap (ODT), and performed 10 milliseconds of non-destructive imaging of the molecules in the ODT. The molecules are then optically pumped into vibrational bending modes and the trap depth is reduced adiabatically by a factor of 3.5. To prepare molecules in a single hyperfine state, the authors used a combination of optical pumping and microwave pulses.
Spin precession in eEDM sensitive states
To perform spin precession in the eEDM-sensitive state, the authors first adiabaticize the electric field to EZ and then turn on the small bias magnetic field BZ. The electron spin precession frequency was measured using a procedure similar to Ramsey energy spectroscopy, tuned by the applied electric field EZ. In order to map the location of the zero-g factor crossover, spin precession measurements were performed for different electric fields under a fixed magnetic field BZ = 110 mG, and it was found that the zero-g factor crossover within the eEDM manifold occurs at an electric field of 59.6 V/cm. The authors emphasize that while the location of these crossovers depends on the structure of the specific molecule, their presence is universal among polyatomic molecules.
Relevant times and restrictions
The authors present measured coherence times at different applied fields BZ and EZ, describing the two main limitations of long-term elimination of oscillations. The authors found that the drift of the bias electric field EZ is negligible in the device and that decoherence caused by magnetic field noise dBZ is independent of the applied magnetic field. At higher magnetic fields, the main limitation on coherence time is the AC Stark shift from the optically trapped light. Intense Z-polarized ODT light causes electric field changes where zero g-factor crossover occurs. According to the research results, it is expected that the longest achievable coherence time will occur at very small g factors (geff≈0) and very small bias fields (BZ≈0).
References
LOÏC ANDEREGG, et al. Quantum control of trapped polyatomic molecules for eEDM searches. Science, 2023, 382(6671):665-668.
DOI: 10.1126/science.adg8155
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